truck wont start? Lift pump vs. air intrusion

[goatwrangler]

hey all,

My truck wont start. 1984 F250 6.9l Automatic. I have bypassed my water seperator mounted on the firewall. I have changed the orings and return lines for the injectors also. When depressing the schrader valve while cranking I only have a little bit of fuel come out. The fuel filter is fuel of ATF. I even checked twice, by taking it off and making sure it was full. I tried starting the truck afterwards and she started but didnt run long. I loosened the supply line from the lift pump to the filter head and have fuel coming out when cranking but very litte fuel out of the schrader valve unless I crack a supply line loose then the air pushes the fuel out. Maybe my fuel filter is blocked ?

I hope this makes sense ?

 

[Hydro-idi]

Your lift pump could be very weak and on it's way out. Check the weep hole on the lift pump. If it is leaking fuel and/or dumping fuel into your crankcase, replace very soon because diaphragm has failed. Also follow up with an oil/filter change. Diesel fuel is ******* engine bearings if there is diesel in crankcase.
I knew my lift pump was shot because it started leaking out of that weep hole. However, it was a fairly easy fix.

 

[goatwrangler]

I dont see any leaks from the weep hole

 

[bbjordan]

I tried the same ATF in the fuel filter trick and got the same result. Fired it up, it ran a bit and then quit. I don't think it liked straight ATF. The engine was warmed up at the time too! I tried starting it quite a few more times and then just let it sit overnight. The following morning after a few attempts it roared to life. I'm sure it just had to dilute the ATF in the filter with fuel. You might want to put it on the battery charger... The IP was good after that.

Start with the filter first, and then the lift pump.

 

[Hydro-idi]

If the lift pump checks out, my guess is that you have air in the fuel lines. A fresh fuel filter would help and your truck will run on atf just fine. I used to run my truck on 100% WATF and it ran good lol. Anyways has your truck sat for a while or did this issue come up all of a sudden?? If truck started and ran for a couple seconds....then died, this is a classic air intrusion issue.

 

[Hydro-idi]

Should note: if it is really cold outside, diesel fuel in the filter may light er off better. Forgot to consider temperature. Diesel fuel is easier to light off than atf when it is really cold outside.

 

[goatwrangler]

I'll get a new filter tomorrow and fill it up with diesel then try her again. I just dont get it right now because my fuel lines are in good shape...

my question though: Why would I have better fuel pressure after cracking a supply line ? Because it sucked in air and the air is pushing the fuel ?

I can pull my supply line off the lift pump and put it in a fuel can full of diesel to check if I have an air intrusion before my lift pump correct ?

 

[bbjordan]

Yup.

 

[goatwrangler]

I replaced the lift pump and the pump, pumps fuel but Im not getting anything out of the schrader valve on the filter head ?

 

[Hydro-idi]

Still sounds like you have an air pocket that won't pass through. I might try cracking open your injector lines slightly and try starting it. This should help in the aid of air removal in your fuel system.

 

[Wyreth]

have you replaced your fuel filter yet?

 

[goatwrangler]

Yep, I replaced the fuel filter and the lift pump.

On my lift pump, the supply line to the lift pump makes a loop so I was thinking that may be holding the air pocket but not sure

 

[Wyreth]

I JUST replaced the lift pump and filter on my rig. The loop didn't hold air, or if it did, it didn't hold it long enough for the filter to run out of fuel. Try pulling the output line from the lift pump and seeing if you're getting fuel there while cranking. If you aren't I think it might be wise to run a line from a fuel can to the input side of the pump and see what happens. The last time I couldn't get fuel it air intrusion before the pump.

In my experience, a held air pocket is more likely after the pump. Rather than before.

 

[goatwrangler]

Wyreth, when the output line is disconnected at the pump, the pump does squirt fuel.

 

[Wyreth]

And you're not getting anything out of the schrader valve on the filter head? Ok well, for a long shot, do you get fuel out of the line at the filter head when you crack it there? If so, how much fuel are you getting? It should be a pint for 10sec of cranking. (a "new" pump doesn't always mean a good pump) If that checks out, are you getting air hissing out the schrader? If not, pull the schrader valve off the filter head, and see if it pumps fuel out the hole. If it does, then you may have an air bubble trapped in the supply line to the IP or injectors. If so, like Hydro said, crack ALL the injector lines at the injectors. Then crank it. Tighten the wet ones down, the crank again. Repeat until it starts, then crank the rest of them back down.


Stupid question... you're not getting white puffs out the tailpipe while you're trying to start it right?